Obsolete since Gecko 2 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
__parent__ property used to point to an object's context, but it has been removed.
For top-level objects, this is the e.g. window.
Not part of any specifications.